The present invention relates to a method of compacting an anodically produced layer of oxide on parts of aluminum or aluminum-alloy.
Aluminum parts, such as aluminum stampings or rolled sections are used, inter alia, in the hardware and lighting industries or, in particular, also in automobile construction, for instance for window mounting systems or else as ornamental frames, ornamental moldings and the like. In particular, there are used aluminum or aluminum alloys parts which have an anodically produced and predominantly also a colored anodized layer. Traditionally, the parts are developed with thicknesses of the anodized layer of about 20 .mu.m since it was assumed up to now that such layer thicknesses are necessary in order to produce, for instance, intensive colorings which provide sufficient protection against corrosion. Up to now, it has also been assumed that anodized layer thicknesses of 20 .mu.m are necessary so that, upon the compacting of the layers, a sufficient absorption of water of crystallization takes place at temperatures of 98.degree. C. to obtain a grid transformation of the layer which closes the pores so that the substances previously absorbed in the layers are firmly enclosed. Anodized layer thicknesses of 20 .mu.m and more are disadvantageous since the degree of luster of the surface is considerably reduced and the optically positive impression is therefore negatively modified. Furthermore it was found that upon mechanical and thermal action, the 20 .mu.m anodized layer can show fine hair cracks. Thus the parts cannot be used since the hair cracks can lead to corrosion. Furthermore, considerably longer dwell times (100%) in the anodizing baths are necessary in order to build up the 20 .mu.m anodized layer. As a result of this, the cost of producing the anodized layer is increased.